PON architecture is being used today by certain telecommunications operators for forthcoming major optical deployments for residential customers. Given the volume of equipment and infrastructure to be installed, the search for minimum investment costs is of particular importance in this sphere.
The choice of the PON architecture reduces investment costs since it enables the pooling of a part of the equipment and infrastructure
FIG. 1 is a simplified drawing of a passive optical network PON 1000. The PON 1000 comprises a central office 100 itself comprising several frames 101, 102 which are optical line terminations or OLTs.
In each frame 101, 102, it is possible to insert several PONs (not shown) comprising one or more physical interfaces, each constituting the starting point of an optical tree structure. Here below, these interfaces are called transmission modules. In the context of FIG. 1, the description pertains to a particular case in which the frames 101, 102, each comprise a single transmission module 1011 and 1021 constituting the starting point of an optical tree structure 110, 120.
Each of the tree structures 110, 120 has characteristics laid down by the different ITU (International Telecommunications Union) standards issued by the FSAN (Full Service Access Network) group, for example the G.983, G.984 recommendations or the IEEE 802.3ah standard.
These standards in particular specify the maximum physical partitioning rate for the optical infrastructure and the constraints on the length of the links between the central office 100 and the user modules, these two variables being dependent because they form part of an optical budget range.
The tree structure 110 comprises for example two coupling levels based on three 1×N couplers (where N is a positive integer, for example equal to 8) 111, 112, 113 to which optical network terminations 114 or ONTs, below designated as user modules, are connected.
The tree structure 120 has two coupling levels based on three 1×N couplers 121, 122, 123 to which optical network terminations 124 are connected.
A management platform (not shown) hosted by a station (for example a PC, a server etc) is connected to the different PON cards of the transmission modules 1011, 1021 of the central office 100 in order to configure the exchanges between the transmission modules 1011, 1021 and the ONT user modules 114, 124 and centralize the different pieces of information on management of the PON 1000.
Flexibility in the time domain is defined in the context of the ITU standards issued by the FSAN group: this is a case of dynamic bandwidth allocation or DBA aimed at dynamically providing extra capacity, in the network intake sense (the uplink direction), to the user modules 114, 124, that ask for it by reusing resources not specifically used by other user modules 114, 124.
A classic scheme of deployment of a PON corresponds to a tree structure comprising two coupling levels, each based on 1×8 cascade-mounted couplers.
In the case of a low density of user modules, the first coupler can be situated at the level of the central office to favor rapid filling of the optical interface to the detriment of the efficiency of partitioning of each fiber which thereafter is shared between only eight user modules.
The trend nevertheless is towards sufficient densification of the user modules, implying that a first coupler is situated at a distance from the central office, for example at the end of a first fiber section or feeder. In this case, the essential part of the route of the fiber is shared, thus providing for substantial economic optimization.
Thus, in the case of great density of user modules, the making of conduits (in public works undertakings) used by the fiber calls for the paving of the entire zone to be connected with a consequent sub-division into plates of this zone to be connected.
The main three parameters that have a direct effect on the number of transmission modules associated with a PON card to be deployed in the central office are:                the distribution of the clients (or user modules) as a function of the configuration of the paths that may be taken by the cables;        the total payload bit rate per optical interface at the central office;        the rate of physical partitioning per optical interface at the central office.        
The effect of the latter two parameters is obvious:                as soon as the aggregate bit rate (or sum of the bit rates) to or from all the user modules exceeds the maximum bit rate that can be sent or received by the transmission modules connected to the central office, it becomes necessary to implement a new transmission module, even if the maximum partitioning rate allowed at the optical budget level is not attained;        as soon as the number of user modules connected reaches the maximum physical partitioning rate of the transmission module, it becomes also necessary to implement a new transmission module, even if the total or aggregate bit rate to or from the user module is lower than the maximum bit rate that can be sent or received by the transmission module.        
The parameter linked to the distribution of the clients as a function of the configuration of the paths that can be taken by the cables has a direct influence on the number of transmission modules to be deployed in the central office owing to the configuration of the conduits and the resulting sub-division into hubs.
Indeed, if clients associated with user modules request a connection and if they are situated in a direction that corresponds to no starting point of an already installed PON card, then it is necessary to install a new transmission module associated with a starting point of a PON card of the central office in order to service them.
Classically, it is sought in principle to optimize the use of the capacities of the transmission modules (whether in terms of partitioning rate, aggregate bit rate or distribution of clients as a function of the configuration of the paths that the cables can take) at the time of the physical installation of the network and possibly at the time of the installation of new transmission modules in the network.
However, since this optimizing is done in principle, at the time of the installation of the network, and since the needs of the users (whether in terms of partitioning rate, aggregate bit rate or distribution of clients, or user modules) as a function of the configuration of the paths that can be taken by the cables change and develop constantly especially after the installation of the network, this optimizing proves to be complicated and inefficient.
Thus, in certain cases, an under-utilization can be seen in the capacities of the transmission modules of the PONs.
There is therefore a need for a technique to overcome these drawbacks of the prior art.
Furthermore, a technique of this kind should make it possible to optimize the use of the capacities (especially in terms of partitioning rate or aggregate bit rate) of the transmission modules of the central office of an optical access network.
A technique of this kind should also be capable of reducing the quantity of apparatuses to be deployed in such a network and therefore reduce the costs and space requirement of the network.
Such a technique should also make it possible to provide greater flexibility in the use of the access network.